This invention relates to a data mapper and method for mapping digital information sequences into a SONET (synchronous optical network) payload in such a manner that there can be an arbitrary mixture of value types, viz. control and data, in those sequences.
A SONET optical data signal (i.e., such as STS-48, meaning 48 synchronous transport streams) is typically composed of multiple STS-1""s which are assigned to various resources or clients, with the greater the number of STS-1""s representing increased bandwidth. An advantageous feature of SONET framing, which makes it particularly desirable for metro and wide area optical transport networks, is that it provides a deterministic and flexible bandwidth allocation.
Each STS-1 of a SONET frame has a frame format consisting of rows and columns of fixed numbers of octet sequences (an octet having 8 bits and being alternatively referred to as a byte) and the first few columns of octets contain transport overhead information while the remaining octets form a payload which transports user information. The payload format for a SONET frame is static and, accordingly, the known methods used for the mapping of data into a SONET frame are also static in that each octet position in the SONET frame payload is assigned a predetermined meaning between the sender and receiver of the mapped data. Because of the nature of the SONET payload format the user information which is normally mapped into a SONET frame is limited to structures having formats in which control and data type information have either fixed positions in the input and output information sequences or can be differentiated on the basis of algorithms previously applied to them such as HDLC byte stuffing.
However, many information sequence structures do not have a fixed structure and instead may contain an arbitrary or varying mixture of control and data type values. For example, this is true of packetized data such as Ethernet frames, IP datagrams, physical layer encoding schemes such as 8b10b as well as other information structures with two information types (i.e., control and data). For these information structures it can be difficult to distinguish between octets containing control information and octets containing data information and this makes it necessary to add checks to the octet sequences to ensure that these information types can be distinguished. For example, for packetized data such as Ethernet transport data, the above-mentioned mapping scheme based on HDLC byte stuffing is often used for delineating frames (i.e., marking frame boundaries so as to distinguish between two frames). This byte stuffing technique defines two control octet values, one used as a frame delimiter code, and the other to mark data codes with the same value as either control code to prevent misinterpretation. When a data code value matches either control code value, the mark data control code is inserted in front of the data code, and the data code value is adjusted.
Such methods are problematic, however, because the number of octets required, and therefore bandwidth required, to transport a data/code sequence is then determined by the data content and as many as twice the number of data structure octets may be required to carry the data structure (since, in theory, it could become necessary to mark each data code). Extending this technique to carry many different control code values, as required for transport of line codes like 8b10b would worsen this problem. The resulting loss of a deterministic bandwidth capability thereby requires additional bandwidth to be provisioned across a SONET network to provide a guaranteed quality of service.
There is a need, therefore, for means to enable a flexible mapping of data into a SONET frame by which control and data information value types need not be assigned to any fixed position in the frame and also need not be distinguished by adding extra check codes.
The present invention provides a data mapper and method for mapping digital information sequences, which may comprise variable, arbitrary mixtures of control and data value types, into a SONET frame.
In accordance with the invention there is provided a digital information mapper for mapping input information characters into a SONET frame synchronous payload envelope (SPE) wherein the information characters comprise control and/or data information and each character comprises 9 bits consisting of an 8 bit information octet, being either a control octet or a data octet, and one octet type bit identifying the octet as control or data. The mapper includes a processor and a mapping module configured for performing a mapping algorithm, wherein the mapping algorithm is applied by the processor to sequences of eight input information characters. The algorithm is operable to map the information octets of each sequence of eight information characters to eight of a sequence of nine contiguous SPE octets and to map the octet type bits for each mapped sequence of information octets to the ninth octet of the contiguous SPE octets of the SPE octet sequence.
In accordance with a further aspect of the invention there is provided a method for mapping input digital information characters into a SONET frame synchronous payload envelope (SPE). The method includes the steps of mapping the information octets for each sequence of a plurality of sequences of eight information characters to eight of a sequence of nine contiguous SPE octets and mapping the octet type bits for each mapped sequence of information octets to the ninth octet of the contiguous SPE octets of the SPE octet sequence.
Preferably, the octet type bits for a sequence of information characters are mapped to the ninth octet of the SPE sequence in such a manner that each octet type bit is mapped to the bit position of the ninth octet which corresponds to the octet position, in the SPE sequence, of the information octet with which the octet type bit is associated.